Why doesn't extruding from some edges make a face while usually it does?
Can you confirm that you select the connected vertices rather than a duplicated one?
You can use indices indicator to show the vertices. Than you should check which vertex you choose to extrude. If you select two separated vertices, they won't automatically connected with face and edge since it is impossible and unusual.

You might accidentally exit the extrude mode with RMB, it will create the vertices in the same place. Causing extra vertices that user might not noticed.